Question: Solve for $x$ and $y$ using substitution. ${3x+3y = -3}$ ${x = 5y-7}$
Explanation: Since $x$ has already been solved for, substitute $5y-7$ for $x$ in the first equation. ${3}{(5y-7)}{+ 3y = -3}$ Simplify and solve for $y$ $15y-21 + 3y = -3$ $18y-21 = -3$ $18y-21{+21} = -3{+21}$ $18y = 18$ $\dfrac{18y}{{18}} = \dfrac{18}{{18}}$ ${y = 1}$ Now that you know ${y = 1}$ , plug it back into $\thinspace {x = 5y-7}\thinspace$ to find $x$ ${x = 5}{(1)}{ - 7}$ $x = 5 - 7$ ${x = -2}$ You can also plug ${y = 1}$ into $\thinspace {3x+3y = -3}\thinspace$ and get the same answer for $x$ : ${3x + 3}{(1)}{= -3}$ ${x = -2}$